Control of process conditions in an electrophotographic apparatus can be provided by forming toned density patches on the photoconductors. Such patches are formed by exposing, for example, interframe portions of the photoconductor to exposure light from the imaging source and developing same with the development station under appropriate electrical bias. By measuring the density of the patches, it can be determined whether adjustments are needed to one of the known operating process control parameters such as primary charger setpoint, exposure setpoint, toner concentration, and development bias.
The density of the developed toned patches can be measured using a densitometer. One type, a transmission densitometer, projects light, visible or infrared, through an object onto a photodiode. The amount of energy reaching the photodiode determines the voltage output from the device.
In a copier/printer, the photoconductor passes between the light source and the photodiode. When the photoconductor has toner on the surface, the amount of light reaching the photodiode is decreased. This changes the voltage output from the device, proportional to the amount of light transmitted through the toner on the surface. Based on this voltage, the amount of toner applied to the photoconductor can be varied as required in order to obtain consistent image quality. Another type of densitometer as described in U. S. Pat. No. 5,519,497 uses reflected flux rather than transmitted flux to determine density.
As these machines are used, age of the photoconductor, degradation of the densitometer light emitter, and contamination on surfaces change the voltage output. Typically, the photoconductor is provided with a calibration process to null out the effects of age and contamination, but even so it may be necessary to remove it frequently in order to clean the photodiode or other surfaces.
Many densitometers known from the prior art convert light to density reading using a logarithmic amplifier, which is an expensive device that significantly adds to the cost of the product. There is also prior art that use a look-up table. The output of a normal amplifier is the input to the look-up table. The amplifier output must be converted from analog to digital format before being applied to look-up table.